[Minor] Enhance JIT kernel and add dev docs (#14570)

python/sglang/jit_kernel/test_add_constant.py

@@ -0,0 +1,14 @@
+import torch
+
+from sglang.jit_kernel.add_constant import add_constant
+
+
+def main():
+    c = 1024
+    src = torch.arange(0, 1024 + 1, dtype=torch.int32).cuda()
+    dst = add_constant(src, c)
+    assert torch.all(dst == src + c)
+
+
+if __name__ == "__main__":
+    main()